The present invention relates to tires for heavy vehicles, such as trucks and buses. More particularly, it relates to the beads of radial carcass tires which have at least one bead wire in each bead and are intended to be mounted on rims defined in accordance with the existing standards and having flanges axially on the outside.
In certain cases, tires for heavy vehicles are called upon to support substantial overloads which produce flexings at the level of the side walls of the tire of an amplitude which is greater, and therefore more disadvantageous, the greater the overload. This problem is also encountered in the case of twin tires when one of the two tires has suffered a loss of pressure, as the result, for instance, of a puncture, and the other tire, which bears the entire load, experiences at its sidewalls flexings which are very disadvantageous for the life of the carcass reinforcement.
Finally, for certain heavy vehicles there is a demand for tires the overall diameter of which is substantially reduced while they retain the profiles and dimensions of rims at present on the market in order to increase the useful load transported; if H represents the height of the tire mounted on its rim, measured on a meridian section between the point of the bead closest to the axis of rotation and the outermost point of the tread of the tire and S the overall width of the tire measured parallel to the axis of rotation, the aspect ratio is defined by H/S.
In the case of aspect ratios less than or equal to 0.6, poor resistance to fatigue of the tire under the cycles imposed by travel is noted; in each side wall of the tire extending between a bead and the belt of the crown, the corresponding portion of the radial carcass is reduced in height and, therefore, each carcass cord undergoes cycles of flexure along small radii of curvature. In operation, upon each revolution of the wheel these cords are subjected to cycles of variation in curvature which are more disadvantageous the smaller this aspect ratio and therefore these radii of curvature.
Various proposals are known which are directed at overcoming excessive fatigue in the sidewall of a tire during the course of travel. Among them, mention may be made of French Patent No. 1,502,689 which discloses that by reinforcing this zone of the tire with, for instance, a layer of rubber stock, the tire is imparted additional rigidity and it is thus possible to decrease the amplitude of the flexing cycles. However, such an arrangement results in an increase in weight and particularly in heating of the sidewalls and therefore in a consumption of energy.
Another proposal disclosed in French Patent Application No. 2,415,016 suggests producing a "depression" in the sidewall of the tire, thus making it possible to reduce the height of the bead and increase the height of the sidewall and therefore to increase the flexibility of the sidewall. This solution makes it possible effectively to increase the life of the sidewalls under strong flexure, but in a manner which is still limited in part due to the fact that the zone of the bead of the tire which is furthest radially to the outside is still forced to flex along the profile of the flange of the rim.
While these two proposals make it possible substantially to increase the life of the sidewall, they still are insufficient in the case of tires of ratios less than or equal to 0.6.